1. Field of the Invention
This invention relates generally to four-stroke spark ignition internal combustion engines and more particularly, to methods and devices for efficiently controlling the power output of a spark ignition internal combustion engine.
2. Description of the Related Art
In a four-stroke internal combustion engine, a single cycle of operation occurs over four strokes of a piston within its cylinder, made during two crankshaft revolutions. Engines having more than one cylinder generally evenly stagger the cycles of the cylinders for smooth operation, with each cylinder completing the four-stroke cycle within two consecutive engine revolutions.
The four strokes of the piston that occur in a single cycle of operation are the intake, compression, power and exhaust strokes. Generally, at the beginning of the intake stroke when the piston is at or near the top of the cylinder, the intake valve opens and the descending piston draws air into the cylinder. The position of the piston at the top of its movement toward the valves is called top dead center (TDC). At or near the bottom or end of the intake stroke, the intake valve closes. The position of the piston at the bottom of its movement away from the valves is called bottom dead center (BDC). From this position, and with the intake and exhaust valves closed, the piston starts upward on its compression stroke until it reaches TDC. The piston compresses the air or air-fuel mixture captured within the cylinder upon closure of the intake valve into a small space at the top of the cylinder and adjacent to the valves. Usually just before the piston reaches TDC on its compression stroke, the spark plug fires to ignite the air-fuel mixture, causing the mixture to burn quickly. The rapidly expanding gases of the burning mixture force the piston down and away from the valves on its power stroke. As the piston reaches the bottom of the power stroke (BDC), the exhaust valve opens for the exhaust stroke of the piston. During the exhaust stroke, the piston moves back toward TDC and forces the combustion products, mainly carbon dioxide, carbon monoxide, nitrogen oxides, and some unburned hydrocarbons, out of the cylinder through the exhaust valve and into the exhaust manifold.
The intake valves and exhaust valves are typically operated by cam lobes on one or more camshafts. The valves are generally spring-biased toward their closed or seated positions. As known in the art, a simple engine may have one camshaft with multiple cam lobes that operate both the intake and exhaust valves, while more complex engines may have multiple camshafts that can operate more valves per cylinder, such as, for example, a double overhead cam arrangement in a four-valve-per-cylinder engine. A cam lobe is rotated by a camshaft to apply force to the top of the valve stem, either directly or through a rocker arm, and operates the valve to open and close as the camshaft rotates. The camshaft rotates in a phased relationship with the crankshaft of the engine, usually through a belt, chain or gears, and is synchronized with the crankshaft so that the valves open and close at the proper times during the engine's cycle. Typically the camshaft is designed to turn at one-half the angular speed of the crankshaft.
Parasitic losses occur throughout an internal combustion engine and are caused, for example, by friction, valve operation, exhaust backpressure and the throttling of intake air across the throttle valve. Such losses decrease the fuel efficiency of the engine, thereby increasing fuel consumption and exhaust emissions that result from the increased fuel consumption. The automobile industry has developed many useful solutions to reduce these parasitic losses, but there still exists an ongoing need to improve engine efficiency and to further reduce losses.
One industry solution to the problem of parasitic losses has been the implementation of optimized camshaft profiles and valve timing. When an engine is operating at low speed, such as when idling, the optimal camshaft profile is close to the “theoretically normal” intake valve opening and closing points, i.e., 0 degrees after top dead center (ATDC) and 180 degrees ATDC on the intake cycle. Such camshaft and valve timing may be considered “slow speed” or “conservative” and results in a steady, smooth, and strong idle.
However, at high engine speeds, usually above 2000 rpm, the optimal intake valve opening time is earlier, for example, 15 degrees before top dead center (BTDC), and the optimal intake valve closing time is later, for example, 220 degrees ATDC. A “high speed” cam profile providing this valve timing maximizes the quantity of air captured within the cylinder and provides more torque and power at high rpm, but also cause rough idling and increased exhaust emissions at low speed.
To optimize an engine's performance both at high and low speeds, systems have been developed that vary the timing of the opening and/or closing of the valves relative to the angular position of the crankshaft by advancing or retarding the valve timing. Other systems have been developed that vary the length of time that the intake valves remain open. Such methods may be implemented separately or jointly.
For example, in U.S. Pat. No. 6,502,536 issued to Lee, et al., which is fully incorporated by reference, a two-step roller finger follower is disclosed that provides for selecting between two separate, fixed cam profiles. Switching between different cam profiles enables a selection of a high lift/long duration or a low lift/short duration operation of an intake valve. Lee discloses that for efficient low-speed operation of the engine, the low lift and short duration cam profile for the intake valves is implemented, and at high speed, the high lift and long duration cam profile for the intake valves is implemented. Lee further discloses that the angular position of the camshaft relative to that of the crankshaft may be varied. Such camshaft control is known as cam phasing.
Cam phasing varies the timing of the opening and/or closing of the valves relative to the angular position of the crankshaft by advancing or retarding the valve timing. Cam phasing may be used to control intake and exhaust valve overlap as a function of engine speed and provides a finer control of the engine's operation by modifying the mechanical linkage between the camshaft and the crankshaft to modify the intake valve profile. Valve overlap refers to the condition in the four-cycle engine when both the intake and the exhaust valves are open at the same time. At idle and low engine load, overlap is at its minimum to improve idle quality, while at higher engine speed and greater load, the overlap is increased to provide higher power. Cam phasing technology can be used to control the overlap event by shifting both the intake valve profile and the exhaust valve profile, one relative to the other, and both relative to the crankshaft.
In U.S. Pat. No. 6,600,989 issued to Sellnau, et al., which is hereby fully incorporated by reference, a method for early intake valve closing (EIVC) is disclosed. Sellnau discloses an intake valve cam phaser that enables optimization of the timing of the intake valve relative to the engine crankshaft. The cam phaser enables the opening and closing of the intake valves to be controlled relative to the rotational position of the crankshaft. Sellnau discloses an EIVC system and method utilizing a two-step cam profile switching device and an intake valve cam phaser to achieve variability in the intake valve lift profile of an internal combustion engine in response to engine operating parameters. Sellnau's EIVC systems shift the entire intake valve profile so that the intake valve closes earlier than it would otherwise close relative to the piston motion; that is, when the engine is in EIVC mode, the intake valves may close before the pistons reach BDC. According to Sellnau, early closing of an intake valve reduces the amount of air and fuel in the cylinder to satisfy the decreased power demand. Sellnau discloses that reducing the amount of fuel used increases engine efficiency and decreases emissions as compared to conventional intake valve operation.
There are many different cam phasers known in the art. For example, U.S. Pat. No. 6,622,677 issued to Simpson, et al., which is fully incorporated by reference, discloses a cam phaser for adjusting the angular relationship between a camshaft and a crankshaft. Simpson discloses a cam phaser having a worm gear mounted on an inner housing meshed with the internal teeth of an outer housing. The inner housing is connected to the camshaft. The worm gear is connected to one or two drive wheels, which are rotated by contact with stationary plates. The plates are moved by electromagnetic coils to contact the drive wheel or wheels, and turn them in one direction of the other.
Another cam phaser disclosed in U.S. Pat. No. 5,203,291 issued to Suga, et al., which is fully incorporated by reference, shows an outer housing containing internal gear teeth, which are turned by small gears. The small gears are driven by a pin on the spiral cam, which is in turn disposed on the gear shaft. A pair of stopper pins is also present to restrict the rotation of the gear when necessary.
While these valve phasing techniques may optimize the timing of valves to the power demand of the engine, they do little to address one of the largest parasitic losses that robs the engine of power and efficiency, especially at low engine speeds. Even cam phasing methods designed to optimize valve operation at low engine speeds do nothing to alleviate power losses attributable to the throttling of intake air across the throttle valve. In conventional spark ignition internal combustion engines, control of engine power and speed is obtained by throttling the intake air to the cylinders using a throttle valve, which is typically a butterfly valve. The accelerator is linked to the throttle valve to control the amount of combustion air that enters the engine cylinders through the intake valves. Engine power output is increased by pushing on the accelerator and opening the throttle valve to allow more air to enter the cylinders, and increased air can be used to burn more fuel per combustion event.
The operation of the throttle valve and the pumping action of the pistons create a vacuum in the intake manifold. When the throttle valve partially closes, a pressure drop is imposed on the air flowing into the intake manifold and across the throttle valve. The vacuum created in the intake manifold limits the amount of air that can be drawn into a cylinder during the intake stroke of the piston, thereby limiting the amount of fuel that can be burned to drive the piston on its power stroke. As the throttle valve closes, the vacuum in the intake manifold increases, and as the throttle valve opens, the vacuum decreases. As used herein, a pressure stated in terms of inches of Hg means absolute pressure unless otherwise stated. A typical vacuum in the intake manifold of an internal combustion engine at mid-range speed is between about 16 to 22 inches of Hg (8 to 14 inches Hg below ambient at sea level).
Using a throttle valve to control the engine speed while generating a vacuum is inefficient and costly because the resulting vacuum wastes power and energy. While the vacuum enables power control by controlling the air charge introduced into each cylinder on the intake stroke, the vacuum also retards the crankshaft rotation by resisting piston movement during its intake stroke. This parasitic power loss is most severe at low engine speed.
The vacuum generated in the intake manifold reduces the power output of the engine because less fuel is being burned and because the engine is forced to produce substantial amounts of vacuum whenever the throttle valve is not wide open. The energy cost of producing this vacuum is approximately equal to the cost of producing an equal volume of compressed air at the same flow rate and at the same differential pressure relative to ambient. Power availability is diminished as a result of the produced vacuum.
What is needed is a method for controlling an internal combustion engine that reduces the parasitic loss resulting from the vacuum generated by pulling the intake air across the throttle valve. Furthermore, an internal combustion engine configuration is needed on which such a method may be implemented.